1. Field of the Invention
This invention relates generally to tension leg platforms, and more particularly to a compact offshore tension leg platform (TLP) and a method for installing the same at a deepwater location.
2. Brief Description of the Prior Art
In the past, tension leg platforms (TLP's) have been designed to include the capability of performing well drilling operations. Furthermore, these platforms have supported the risers and well trees at the deck level. To meet these requirements, the platforms have to be large enough to provide buoyancy to support the payload of the well drilling equipment and the risers support system has to incorporate mechanisms which ensure the tension in each riser remains constant as the length of the risers varies due to movements of the hull caused by wave and environmental loadings. The size and complexity of these TLP's have resulted in costs which are too high for the commercial development of small offshore hydrocarbon reservoirs.
In the search for a means of producing from small reservoirs, various small TLP configurations have been proposed. TLP's without any drilling capability have been proposed. All production drilling activities have to be performed by a drilling vessel positioned alongside or over the TLP. The purpose of these platforms has been only to support the risers and position the well trees above the water surface. Production equipment and facilities have been provided on the separate drilling and support vessel.
Other proposed small TLP's have included the provision of production facilities. However, no capability for drilling operations has been included. In summary, all the proposed small TLP's have a requirement for a separate vessel in order that operations can be performed on the wells during the phase of producing from the reservoir.
The present invention provides all drilling capabilities needs for production well completions, re-completions, workover and fluid injection operations. The technical complexity of the large TLP's is, however, avoided by minimizing the above water payload. Moreover the novel hydrodynamic configuration of the platform hull reduces the hull motion responses to wave and current loadings and thereby reduces the loadings in the anchored tendons. Furthermore, if adopted, the placement of the well trees below the water surface simplifies the risers support mechanisms. Installation of the foundations, tendons and hull does not require a specialist deepwater cranevessel and equipment.
The present platform may be used not only for production from small deepwater offshore reservoirs but also, by installing multiple platforms in accordance with the present invention allows a phased development of and production from large reservoirs.
It is therefore a general object of the present invention to provide an offshore tension leg platform and method of installation which will obviate or minimize the complexities and overcome the limitations of conventional tension leg platforms thereby reducing the cost of the platform and of producing from small offshore reservoirs.
It is another object of the present invention to provide a seabed structure which will position the wells and anchor piles in a desired arrangement that may be installed by a conventional deepwater drilling vessel and using normal drilling equipment, and does not constitute an integral part of the installed platform structure.
It is another object of the present invention to provide an offshore tension leg platform wherein the foundation anchor piles are installed using conventional drilling techniques and each tendon is connected to a designated pile, and the anchor piles may be installed prior to, during or after the operations of drilling the production wells.
It is another object of the present invention to provide an offshore tension leg platform wherein the tendons may be manufactured using standard oil industry drilling materials and components and may be assembled and installed by a conventional deepwater drilling vessel.
It is another object of this invention to provide an offshore tension leg platform wherein each tendon is connected to a foundation pile using a conventional flex joint tubular of varying diameter and thickness and the tendon and pile may be varying diameter and thickness and the tendon and pile may be assembled and installed as a single unit and all the platform tendons are installed, connected to the foundation anchor piles before the hull structure is towed to the site.
It is another object of this invention to provide an offshore tension leg platform having a hull structure that is hydrodynamically configured to minimize the magnitude of and the variation of the loadings in the tendons, thereby minimizing the size and complexity of the anchor piles, the connections between the piles and tendons, the tendon bodies and the connections of the tendons to the platform hull.
It is another object of this invention to provide an offshore tension leg platform having a hull structure sufficiently small and simple such that it may be manufactured by the majority of the fabrication facilities which provide such services to the offshore energy industry, and which may be towed, floating, by a conventional towing vessel from the site of manufacture to the offshore designated platform location and connected to the tendons without any need for auxiliary buoyancy elements or cranevessel lifting assistance, and wherein connection of the hull to the tendons is performed simultaneously on the full complement of tendons without the need for complex tensioning devices, slipnuts and other mechanisms.
Another object of this invention is to provide an offshore tension leg platform which supports the well risers of well trees located below the water surface near the keel of the hull at an elevation close to that of the tendon upper connections wherein the means of support is maintenance free and devoid of any hydraulic or hydropneumatic mechanisms, and the well trees have conventional above-water surface components and are devoid of complex fixtures and controls used for conventional subsea trees.
Still another object of this invention is to provide an offshore tension leg platform wherein the platform above surface equipment and facilities are capable of all production operations, with the exception of actual drilling of the wells, autonomously and without the need for supplementary vessels and equipment, and if so desired, may have equipment and facilities which permit the transfer of fluid products to a floating tanker vessel for exportation from the site, or by means of a conventional pipeline, and may be equipped with a means of mooring the tanker, if such means is used.
It is a further object of this invention to provide an offshore tension leg platform wherein the major portion of the platform components may be used again at a different site, including the deck, all equipment there on, the hull, the risers and well trees, and the tendons.